1. Field of the Invention
The present invention relates to a logic gate and logic circuit that use transistors of semiconductor elements.
2. Description of the Related Art
The higher integration of semiconductors in recent years has been accompanied by the problem of increase in current consumption. There is currently a trend toward decreasing the power supply voltage to reduce the current consumption. However, the performance of transistors does not necessarily keep pace with reductions in the power-supply voltage. Specifically, reducing the power-supply voltage necessitates the reduction of the threshold voltage of the transistors, but the threshold voltage cannot keep up with the reduction in power-supply voltage, and as a result, a reduction of the power-supply voltage brings with it a drop in transistor performance. Various designs are now being used to simultaneously realize a reduction of current consumption and higher speed.
The use of transistors having lower-than-normal threshold voltages to attain higher speeds raises the problem of increase in the OFF current (sub-threshold leak current) of transistors. This sub-threshold leak current is a current that consumes energy even when the circuit is not active (during standby), and this current must be reduced to zero in devices such as DRAM. As a result, transistors having an excessively low threshold voltage cannot be used, and the attainment of high speeds at low voltage becomes problematic.
Methods have been proposed for employing various circuit schemes to constructively use these low-threshold transistors and achieve high-speed circuit operation at low voltage. One such method is disclosed by Sakurai Takayasu in “Low Power Design of Digital Circuits” (International Symposium on Key Technologies for Future VLSI Systems, January 2001, pp. 1-5) and JP-A-2001-143477 (hereinbelow referred to as Patent Document 1).
FIGS. 1A and 1B show an example of a circuit of the prior art that uses low-threshold transistors. In the circuit shown in FIG. 1A, a low-Vt CMOS logic unit is formed with a low threshold voltage for the purpose of reducing the sub-threshold leak current and achieving high-speed operation at low voltage. A PMOS transistor (MP7) is interposed between the power-supply voltage VDD and virtual VDD that is the actual power supply of the low-Vt CMOS logic unit. In addition, an NMOS transistor (MN7) is interposed between power-supply voltage VSS and the virtual VSS that is the actual power supply of the low-Vt CMOS logic unit. These transistors turn ON during active intervals and are thus connected to VDD/VSS and turn OFF during cut-off (standby) intervals. Even when low-Vt CMOS logic unit is made up by low-threshold voltage transistors, this cut-off operation cuts the sub-threshold leak current during standby intervals by isolating the low-Vt CMOS logic unit from VDD/VSS. As a result, a sub-threshold leak current is generated only during active intervals.
FIG. 1B shows a circuit realized by means of another example of the prior art. The symbol “H” in the figure indicates high-threshold transistors, and the symbol “L” indicates low-threshold transistors. In the circuit shown in FIG. 1B, when the state of input is fixed and the input signal IN is “high” during standby, transistors that are OFF are configured with high threshold voltages. As a result, the transistors for cut-off (hereinbelow referred to as simply “cut-off transistors”) are no longer needed. However, when the input signal IN is “low,” the cut-off transistors become necessary.
The following two cases can be considered as the configuration of the cut-off transistors. In one case, the cut-off transistors are made up by high-threshold transistors that have a sufficiently small sub-threshold leak current. In the other case, the cut-off transistors are made up by low-threshold transistors and a negative voltage is applied to the gate voltage. In either case, operating the low-Vt CMOS logic unit at high speed necessitates an adequate increase of capabilities. Adopting a gate size that is three times the normal total block gate length retards the speed of the block unit by approximately 10%.
However, the cut-off circuit for decreasing the sub-threshold leak current that is disclosed in Patent Document 1 has the problems described hereinbelow.
As the first problem, when the low-Vt CMOS logic unit is operating, turning ON/OFF the transistors interposed between the power supply VDD/VSS and the virtual VDD/VSS controls active/standby, whereby the power supply level of the virtual VDD/VSS drops depending on the capability of these transistors. The Vds is therefore degraded in proportion to this drop, and the speed is thus retarded (FIG. 2). In other words, the speed of the block is necessarily lowered, and the sufficient capability cannot be obtained despite the use of low-threshold transistors in the configuration.
Minimizing the loss of speed brought about by the drop in virtual power supply as a countermeasure for this problem necessitates an increase of the size of the cut-off transistors. As a second problem, increasing the size of the cut-off transistors increases the gate charge/discharge current of the cut-off transistors and thus increases the current consumption. The increase in the size of the cut-off transistors is further a cause for the enlargement of the chip size due to increase of the gate surface area. In addition, the cut-off operation causes the amount of charge in the low-Vt CMOS logic to be reset, and further, causes the output wiring to be reset, whereby the current consumption increases further, canceling any current-reduction effect as a result.
Recovery to the operating state from standby as described above necessitates the above-described charge/discharge of the wiring and the gates of the cut-off transistors. The third problem is the sheer size of the capacitance of the gates and wiring, resulting in an increase in the time required for charge/discharge. Not only is the charge/discharge current for the output signal wiring large, but more than several hundred ns are required to return from the cut-off state to a state that allows operation. The problem therefore arises that operation following recovery is consequently retarded.